Somebody at T-Mobile USA is having a not very good day. Actually, make that lots of somebodies.

Employees at the Bellevue-based wireless company, having only recently recovered from the surprise of AT&T’s proposed $39 billion acquisition, have now been informed that T-Mobile miscalculated many of the severance estimates designed to help persuade them to stick around in the meantime.

Miscalculated, as in overestimated. As in, if the affected employees are ultimately laid off as a result of the deal, they won’t be getting as much as the company first told them. The problem was explained in an email to employees, obtained by GeekWire, from Larry Myers, the company’s “chief people officer.”

An excerpt …

Due to an unrecognized data error in a master source file, these severance estimates were based on an incorrect hire date of October 17, 1995. Because date of hire is a key factor in calculating severance estimates, and because only a small percentage of employees have continuous service dating back to October 1995, the severance estimate that some employees received in their Continuity Statement is higher than their actual severance benefit under applicable guidelines.

I personally apologize for this error and for any confusion it has caused. The HR team has worked tirelessly since the March 20 announcement to deliver a high quality product for 40,000 employees, which personifies the attention to the individual employee that is the hallmark of the T-Mobile culture. Both this intent and the level of effort have been extraordinary, but that does not minimize the reality of the calculation error for the severance estimates of a large number of employees.

He goes on to explain that employees will receive corrected notices via email and traditional mail at their homes, if their original statements were miscalculated.

The message doesn’t quantify the size of the miscalculation, or the average discrepancy between the previous estimate and the reality. We’ve asked a T-Mobile representative for comment, and we’ll update this post depending on the response.